48 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



This has been proved by the later work upon the optically active 

 polypeptides composed of these amino acids. 



Only one product can result when the two components consisting 

 of the pure optically active amino acids are combined together, e.g. 

 d-alanyl-d-alanine from d-alanyl chloride and d-alanine ester. 



Two products result when one of the components is optically 

 active and the other racemic. The various combinations of optically 

 active tyrosine and aspartic acid with racemic leucine, alanine, etc., 

 come into this category ; they are designated as, e.g. 



dl-alanyl-l-tyrosine 

 glycyl-dl-lcucine 



These compounds are not optical antipodes and can be separated 

 by simple crystallisation. The separation of the leucyl-asparagines, 



d-leucyl-1-asparagine 

 1-leucyl-l-asparagine 



has been effected, but in the majority of cases no separation was carried 

 out. The similarity of the isomers is so great that they conform to 

 the condition termed by Fischer in 1894 "partial racemism". It 

 occurs almost always when a racemic compound in combination with 

 an active residue cannot be separated into its two isomeric forms by 

 simple crystallisation. 



Cystine is peculiar and resembles the tartartic acids in its stereo- 

 chemistry as its constitution shows, 



COOH . CHfNHJ . CHj . S . S . CH 2 . CH(NH 8 ) . COOH. 



It is composed of two exactly similar halves, and it matters very little 

 with which amino group combination is effected, but if it be combined 

 with two molecules of a racemic acid chloride, e.g., a-bromopropionyl 

 chloride, three isomeric optically active compounds can result, 

 namely : 



d-bromopropionyl-d-bromopropionyl-cystine 



1-bromopropionyl-l-bromopropionyl-cystine 



d-bromopropionyl-l-bromopropionyl-cystine. 



A yield of 71 per cent, of dibromopropionyl-cystine was obtained by 

 Fischer and Suzuki and was apparently a definite substance. It was 

 therefore regarded as the dl-compound, since its formation is indepen- 

 dent of the formation of the dd- or 11-compounds which most probably 

 would result in equal amounts. 



